For some time it has been common to manufacture drinking cups, other containers and plastic sleeves for bottles from expanded thermoplastic materials. A popular material currently in use for containers, etc. is expanded oriented polystyrene. A very popular container of this type is a cup that is molded directly from expandable polystyrene beads in a steam chest. However, cups formed in this manner must have a sidewall that is quite thick in comparison to, for example, paper. The added thickness of a cup formed by the molded steam chest method does not lend itself to a small stacking height, thus more space is required for a stack of a given number of cups. Then too, the inherent mode of manufacture of the steam chested cup prevents it being decorated to any degree until it is completely formed. The decoration of completed cups requires printing techniques that are slower and more expensive than flexographic and other sheet printing techniques employed on sheet stock which is preprinted prior to incorporation into containers or for sleeves used as a protective overwrap on glass containers such as bottles.
This invention is intended for use on plastic sleeve forming machines of the type disclosed in U.S. Pat. No. 3,970,492 issued to S. W. Amberg, et al on July 20, 1976, and U.S. Pat. No. 3,802,942 issued to S. W. Amberg, et al on Apr. 9, 1974. These machines are used to form sleeves from thermoplastic material by forming a rectangular sheet of material into a tubular shape having overlapping end portions and sealing the end portions together. The completed sleeves are then used to form either a thermoplastic cup or a shrink wrap covering for a glass container. In both of the above mentioned U.S. patents the seam is formed by heating the overlapping end portions of a formed tubular shape to soften their facing surfaces and pressing the end portions together to form a seam. The pressing is done by means of bar and results in a slight indentation in the area where the bar contacts the plastic material. The pressing action causes some distortion in the cellular structure of the thermoplastic material, with a resultant deterioration of insulative properties in the seam area. In U.S. Pat. No. 4,013,496 issued to S. W. Amberg on Mar. 22, 1977, the seam is formed by means of a spring loaded roller which presses against the overlapping edges of the tubular sleeve. Since the roller which forms the seam is spring loaded and the seam is formed basically by a pressing action, the thickness of the seam may be different from that of the remainder of the formed sleeve.